This invention is in the field of wireless communications, and is more specifically directed to the wireless transmitting of data packets in an environment containing possible interfering communications.
Wireless local area networks (LANs) have become increasingly popular in recent years. Typically, wireless LAN installations include an access point sited within the vicinity of the various client workstations. The access point, which is typically a network element coupled to a computer workstation by way of Ethernet cabling or the like, serves as a hub for wireless devices within its communications range. Bidirectional communications are carried out between the access point and wireless network-enabled devices that are in range (typically on the order of 100 m), enabling the wireless devices to communicate with one another, with other computers resident on the same wired network as the access point, and with remote computers over the Internet.
Under current wireless networking technology and standards, an example of such being the IEEE 802.11b standard, the wireless communications are packet-based, in that each transmission is transmitted in the form of multiple packets. By being packet-based, the packets need not be transmitted or received in sequence, and will generally not be contiguous in time. Indeed, as known in the art, packets that are corrupted in transmission are retransmitted later in time. Upon receipt of all of the packets for a communication, the receiver resequences and combines the packets into a coherent message. In the 802.11 context, each message packet typically includes a preamble and header portion that contains control information and also information identifying the packet (identifying the message, the sequence of the packet in the message, source and destination nodes, etc.), and also includes a payload portion that contains the actual data being communicated, along with a checksum by way of which errors in the payload portion can be detected and possibly corrected.
Modern wireless networks typically operate in the unlicensed Industrial, Scientific, and Medical (ISM) band which, as known in the art, includes frequencies from about 2400.0 MHz to about 2483.5 MHz. Conventional 802.11 transmission s are signals according to the QPSK and BPSK constellations that are modulated into a “channel” within the ISM band having about a −20 dB bandwidth of about 16 MHz, and providing data rates that can reach up to about 11 MHz. Other wireless devices also communicate in this band. An example of such devices are the newly-popular “Bluetooth” devices, which transmit in a frequency-hopping manner within the ISM band. More specifically, Bluetooth transmissions are carried out in channels that are about 1 MHz in width (−20 dB) that change frequency periodically (e.g., about every 625 μsec).
Considering the likelihood that both 802.11 and Bluetooth devices may be operating within the range of the 802.11 access point, and also considering other ISM transmissions such as wireless telephones, garage door openers, and the like, signal interference can often occur. If two different transmissions occur at the same time in the same frequency channels within the ISM band, typically both transmissions will be corrupted. Accordingly, those in the art have studied ways to reduce the incidence of collisions in this unlicensed band.
Fragmentation is a conventional approach to reducing the packet error rate due to interference. In general, fragmentation enforces an upper limit on packet length, thus reducing the likelihood that an interfering signal will occur within the packet. Typically, under the 802.11b standard, a parameter is used to set the number of payload data bytes transmitted in a packet for a given bit rate, which thus sets the packet length (or fragmentation level). As is fundamental in the art, because interference along any portion of the packet will corrupt the entire packet, longer packet lengths generally result in a higher probability of packet error due to interference, for a given level of interference. Increasing the fragmentation of the transmission, by using shorter payload portions in each packet, therefore provides a reduced packet error rate. However, because of the existence of a certain amount of overhead associated with the transmission of each packet the average throughput rate decreases as the packet lengths are decreased. Examples of such network overhead include packet preambles , packet headers and spacing between packets.
This tradeoff between packet error rate and overhead makes the selection of a fragmentation in a wireless network an important constraint on the overall network performance. Making this selection even more difficult is the nature of the interferers that are now commonly present within a wireless network signal range. Many potential interferers, such as wireless telephones, garage door openers, and the like, may interfere only within certain times of operation. Other interferers, such as frequency-hopping transmissions in a Bluetooth network, further complicate the fragmentation selection, considering the ephemeral nature of the transmissions in the various channels. It can therefore be quite difficult to select a fragmentation level or a transmission channel to maximize the average throughput rate.